US-led coalition spokesperson Colonel John Dorrian said the reinforcements "are temporary forces", adding that the long-term strength would remain at 500. (Representative image) Photograph: (AFP)
The reinforcement means the US now have 900 of its troops on the ground in Syria
The United States deployed additional troops to drive out Islamist State from their stronghold Raqa, Syria.
The US-led coalition announced they had deployed another 400 troops on the ground in Syria.
At the moment, the US have 500 troops fighting in Raqa, as Iraqi forces advanced on the IS group in west Mosul on Thursday.
US-led coalition spokesperson Colonel John Dorrian said the reinforcements "are temporary forces", adding that the long-term strength would remain at 500.
The additional deployment comes as the US announced they would host a meeting of the 68 members of the coalition fighting IS on March 22.
The American military buildup comes amid calls by President Donald Trump for new plans to accelerate the pace of the war against the Sunni Muslim extremists.
IS jihadists are facing simultaneous offensives in northern Syria by government forces, Turkish-backed rebels, and a US-supported alliance of Kurdish and Arab fighters.
In the latest sign they are feeling the squeeze, their chief Abu Bakr al-Baghdadi was reported to have abandoned Mosul, leaving local commanders behind to fight the Iraqi forces.
"He was in Mosul at some point before the offensive.... He left before we isolated Mosul and Tal Afar," a town to the west, said a US defence official.
"He probably gave broad strategic guidance and has left it to battlefield commanders."
Baghdadi, who declared IS's cross-border "caliphate" at a Mosul mosque in 2014, in an audio message in November urged supporters to make a stand in the city rather than "retreating in shame".
Meanwhile, Iraqi forces launched an offensive to retake west of Mosul, after having regained the eastern part of the city last October.
In recent days Iraqi forces have retaken a series of neighbourhoods and a museum where IS militants filmed themselves destroying priceless artefacts.
Iraq's Joint Operations Command announced Thursday the elite Counter-Terrorism Service recaptured the Mualemeen neighbourhood in west Mosul.
(WION with inputs from AFP)